The conversion of nitrogen and hydrogen under high pressure is one of the oldest commercial chemical processes. This process consists in reacting a hydrogen/nitrogen mixture over a catalyst at elevated temperatures and pressures. The ammonia synthesis reaction is exothermic and the high temperature in the reactor is sustained by the heat of reaction through the use of indirect heat exchangers in which the feed stream and the product stream are put into indirect heat exchange relationship. Computers have been proposed in the area of ammonia synthesis plant control.
Under a given set of operating conditions, the ammonia production and the corresponding heat developed are at a maximum for a specific temperature. Both above and below this temperature, the amount of ammonia produced is smaller and, correspondingly, the amount of heat developed is smaller. If the feed stream is preheated in order to bring it to reaction temperatures by the heat developed in the ammonia synthesis process, this operation cannot be carried out in a stable manner below the "blow-out" temperature. At a temperature below this "blow-out" temperature, the heat developed by the reaction is smaller than the heat consumed, thus the reaction is quenched and finally stops.
The temperature at which the maximum conversion occurs and the temperature below which the process described is not stable are not fixed temperatures but depend upon various process parameters, the pressure and the feed stream composition being two of the more important ones.